1. Field of the Invention
The invention is related to the field of linear actuators and in particular to compact electrically driven linear actuators for positioning welding guns or other tools on automated assembly lines.
2. Description of the Prior Art
Electrically driven linear actuators are well known in the art and are used in a variety of applications, such as positioning welding guns or other tools on automated assembly lines, or controlling the position of flaps on aircraft and various other applications. The electrically driven linear actuator conventionally converts the rotary motion of an electric motor to a linear motion using a nut and threaded rod. Henschke in U.S. Pat. No. 3,402,308 and Mabie et al in U.S. Pat. No. 4,579,012 teach linear actuators in which a threaded rod is linearly displaced by the rotation of a nut by an electric motor. In both of these linear actuators, the rotation of the threaded arm is inhibited by the object to which it is attached. This same concept is also taught by Grimm in U.S. Pat. No. 4,603,594 and Carlson et al in U.S. Pat. No. 4,773,497. Paine et al in U.S. Pat. No. 3,660,704 discloses a linear actuator in which an extendable arm is prevented from rotating. A portion of the extendable arm, axially displaced from the threaded portion, has a rectangular cross section which passes through a mating rectangular aperture attached to its housing.
In the alternative, Derby in U.S. Pat. No. 4,919,586 and Schwob in German Patent 3743341 A1 disclose linear actuators in which a threaded shaft is rotated by an electric motor and a nut is linearly translated along the threaded shaft. In the linear actuator taught by Derby, the nut is directly attached to a fixture plate which is guided parallel to the threaded shaft by a pair of parallel guide bars. In the linear actuator taught by Schwob, the threaded rod is disposed inside of an extendable arm and is rotated by a drive belt connected to the output of a bi-directional electrical motor. The ball nut is attached to the internal end of the extendable arm and is linearly displaced by the rotation of the threaded shaft.